"Do you like to swim?"
Translation:Schwimmst du gern?
The verb 'mögen' can only be used to talk about liking nouns; people, places, and things. For example:
Ich mag deine Schuhe. - I like your shoes. Mag sie das Buch? - Does she like the book? Sie mögen der Wein. - They like the wine.
'Mögen' cannot be used to talk about liking verbs; activities and things you do. In this case, you use 'gern' or 'gerne' (completely interchangeable, which one you use is just down to personal preference), which roughly means 'gladly' according to Duolingo's Tiny Cards course, to show when you like an activity. For example:
Ich renne gern(e). - I like to run. Isst du gern(e)? - Do you like to eat? Er schläft gern(e)! - He likes to sleep! Sie trinken gern(e) Kaffee. - They like to drink coffee.
I hope this helps!
Objection. You can use "mögen" with an activity. Even the Duden says that it is OK. Maybe there are regional differences. For me "ich mag (das) Schwimmen" is absolutly OK. (I am German) "Ich möchte jetzt backen", "Ich mag was essen" ("I like to bake [sth.] right now", "I want to eat something") are regular sentences in German.
Pick whichever one you like -- they both mean exactly the same thing.
A bit like "till" versus "until" in English -- exact same meaning and it's a matter of preference which one you use.
Exception: I'd use only gern in the fixed expression gern geschehen. But otherwise I'm not aware of any rule for when one form or the other would be preferred.
Because it's nonsense.
gern is not a verb; gernst does not make any sense.
It would be as if you said "He funs swimming" instead of "He finds swimming fun".
"Do you like to swim?" has to be Schwimmst du gern? -- German uses a different construction from English here, using an adverb to modify the verb "swim" rather than using a separate verb such as "like" or "enjoy".
"Magst du es zu schwimmen?" - Do you like to swim? Is correct but it said it's not right in the app. My German friends say that, "Schwimmst du gern," is more like if you ask the person if they enjoy swimming, not if they like it. And that "Magst du es zu schwimmen," is the more proper used translation.